1. Physics 211 Lecture 4 Today's Concepts: Newton’s Laws:
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Physics 211
Lecture 4
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Today's Concepts:
Newton’s Laws:
Acceleration is caused by forces
Force changes momentum
Forces always come in pairs
Good reference frames

2. The directions of force, velocity, and acceleration
Stuff you asked about:
The directions of force, velocity, and acceleration
Nothing, lets go on to Rotation!!
Third newtons law
i understood it all
Inertial/non-inertial reference frames.
none. This is old material. I did 98% of this in high school: We just didn't call things derivatives. But I would be interested in an example where the mass of an object changes.
This actually isn't something that was discussed in the prelecture, but something that has always bothered me is the derivation of the centripetal force on a banked ramp (it's inverse tangent of v^2/r or something along those lines.
centrifugal force

3. Acceleration is caused by force.
A bigger mass makes this harder

4. Preflight/Act Vote again
Suppose the net force on some object is parallel to the x axis. Which of the following statements best describes the motion of the object:
Its velocity is parallel to the x axis
Its acceleration parallel to the x axis
Both its velocity and its acceleration are parallel to the x axis
Neither its velocity or its acceleration need be parallel to the x axis
B) Acceleration is in the same direction as the net force, but the net force tells us nothing about its velocity.
C) acceleration is in the same direction as the force, and velocity is the same direction as acceleration, so all must be parallel
Vote again

5. Preflight/Act Your preflight results:
Suppose the net force on some object is parallel to the x axis. Which of the following statements best describes the motion of the object:
Its velocity is parallel to the x axis
Its acceleration parallel to the x axis
Both its velocity and its acceleration are parallel to the x axis
Neither its velocity or its acceleration need be parallel to the x axis
Your preflight results:

6. Same acceleration, so larger mass has larger net force.
Act
A force F is applied to a small block, that pushes a larger block. The two blocks accelerate to the right. Compare the NET FORCE on the block w/ mass M, to the net force on the block with mass 5M.
A) FM < F5M
B) FM = F5M
C) FM > F5M
Net Force
Same acceleration, so larger mass has larger net force. 5M a F M

7. Preflight/Act
You are driving a car with constant speed around a horizontal circular track.
The net force acting on your car
Points radically inward toward the center of the circular track
Points radically outward, away from the center of the circular track
Points forward in the same direction your car is moving
Points backward, opposite to the direction your car is moving
Is zero.

8. A) the acceleration is pointing in, so the force must too .
B) the force points outward, explaining why your body shifts to the right when making a left turn.
E) Because there is no acceleration, the force is 0. .
You are driving a car with constant speed around a horizontal circular track.
The net force acting on your car
Points radically inward toward the center of the circular track
Points radically outward, away from the center of the circular track
Points forward in the same direction your car is moving
Points backward, opposite to the direction your car is moving
Is zero.

9. Preflight/Act
You are driving a car with constant speed around a horizontal circular track.
The net force acting on your car
Points radically inward toward the center of the circular track
Points radically outward, away from the center of the circular track
Points forward in the same direction your car is moving
Points backward, opposite to the direction your car is moving
Is zero.
Centripetal acceleration is always pointed inward, and the net force points in the same direction as acceleration.

10. Aside: Centripetal acceleration and force
1) Objects moving in a circle always have a component of acceleration, called centripetal, which is toward the center of the circle.*
2) Centripetal acceleration must be caused by a force:
- Friction, gravity – whatever force keeps it moving in a circle.
- This force is often called the “centripetal force”
3) There is no “new” kind of force here.
4) There is no such thing as centrifugal force.
Wineglass demo
* They can have also have tangential acceleration if their speed is not constant

12. Tablecloth & Dishes

13. Preflight/Act
You are driving a car with constant speed around a horizontal circular track.
The momentum of your car
Points radically inward toward the center of the circular track
Points radically outward, away from the center of the circular track
Points forward in the same direction your car is moving
Points backward, opposite to the direction your car is moving
Is zero.
The momentum points in the same direction as the velocity, which is forward.

14. Fake Forces: Coriolis (YouTube)
Ice-puck
Fake Forces: Coriolis (YouTube)

15. This effect can make cannons miss their target if you don’t take it into account.
Hurricane Catrina
And makes hurricanes rotate CCW (CW) in the Northern (Southern) hemisphere.

16. Forces come in pairs!
Fire-cart

17. They experience the same force.
A small guy and a large football player moving at the same speed collide head-on. Which person experiences the larger force during the collision?
The small guy.
The football player.
They experience the same force. 17

18. The accelerations are the same.
A small guy and a large football player moving at the same speed collide head-on. Which person experiences the larger acceleration during the collision?
The small guy.
The football player.
The accelerations are the same. 18

19. The small guy experiences the larger force.
A small guy moving at a high speed collides with a stationary large football player. Now, which person experiences the larger force during the collision?
The small guy experiences the larger force.
The football player experiences the larger force.
Both experience the same force. 19